Thirty-five respiration-deficient mutants of Chinese hamster cells have been sorted into seven complementation groups, and one unique mutant was found to fail to complement the mutation in two different complementation groups. Mutants in these three groups seem to be related in that they all seem to be defective in NADH-CoQ reductase (complex I of the electron transport chain). This relationship is to be explored further in genetic experiments (somatic cell hybrids of hamster and mouse or human cells), and biochemical experiments (further analysis of mitochondrial functions). Another project deals with a temperature-sensitive cell cycle mutant. It appears that the expression of at least one enzyme activity - ornithine decarboxylase is blocked at the nonpermissive temperature after mRNA transcription and polyadenylation. The molecular mechanisms of this specific defect in a presumed translational or posttranslational event is to be further explored.